Anthelmintic resistance

There are several different parasitic worms that can affect horses and ponies. Horses consume the worm larvae, which can be present on grass, and the adult worms will then emerge within the horse. The adults produce eggs which are excreted back into the grass – creating a cycle. A high worm burden can cause damage to the gut and can be fatal. Wormers aim to break the worm’s life cycle by killing the adult worms living in the horse. However, due to genetic mutations, all species of worm have begun to show levels of resistance to wormers.

Our approach

Here at The Mare and Foal Sanctuary we have seen the detrimental effect that heavy worm burdens can have on horses and ponies. That’s why we took a new approach to worming the horses in our care.

In 2021 we started faecal worm egg counting (FWEC) our horses four times a year. Based on veterinary advice, we only worm those equines with a FWEC of greater than 350 eggs per gram. Since taking this approach we’ve seen a reduction in the number of horses we’ve needed to worm, but also a reduction in horses with high worm burdens (over 600 eggs per gram). In 2023 we’ll be comparing FWECs of horses of different ages and those with underlying health conditions. By looking at these factors we can begin to assess whether resistance to wormers is present in our herds.

Sharing our knowledge

The threat of anthelmintic resistance affects every horse and every owner. The decisions we make regarding wormers have a direct impact on horse health and the wider equine community. That is why it’s essential that we share the findings of our research and continue to promote best practice. So far, we have:

become a member of the wider stakeholder group Controlling Antiparasitic Resistance in Equines Responsibly (CANTER), coordinated by the Veterinary Medicine Directorate
presented our findings through a poster at the Horses Inside Out conference. You can view the poster here.
held a free webinar with British Equine Veterinary Association (BEVA) President-Elect David Rendle about wormer resistance where he shared his own research.

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Campaigns and Projects

From stamping out strangles to fighting anthelmintic resistance, we adopt and support evidence-based research and campaigns focused on improving the lives of horses and ponies and the humans who live and work with them.